Braking system for an agricultural vehicle

ABSTRACT

An agricultural vehicle is disclosed having ancillary equipment that may be raised and lowered relative to the ground with consequential changing of the position of the center of gravity of the vehicle and having a hydraulic braking system comprising a master cylinder and a slave cylinder. The braking system further includes a pressure relief valve for limiting the hydraulic pressure applied to the slave cylinder only when the vehicle is in road mode. This ensures that the braking force does not exceed a safe limit below which the vehicle does not risk toppling forwards when braking.

FIELD OF THE INVENTION

The present invention relates to a braking system of an agriculturalvehicle and is applicable in particular to vehicles, such as combine andforage harvesters, having ancillary equipment that may be raised andlowered relative to the ground with consequential raising and loweringof the centre of gravity of the vehicle.

BACKGROUND OF THE INVENTION

On combines and forage harvesters, as well as on other agriculturalvehicles, brakes are used to turn the vehicle within a smaller turningcircle than would be achievable by the use of the steering wheels. Thebrakes are also the preferred method of steering in difficult fieldconditions.

In order to achieve this, two separate braking circuits are providedwhich have separate brake pedals for braking the left and right sides ofthe vehicle. The brakes are designed to be very powerful so thatsteering using the brakes can be achieved with minimal effort.

Furthermore, in combine and forage harvesters the hydrostatic drivesystem is often used for braking. During field operation, thehydrostatic drive system serves as the primary means for stopping thevehicle while the friction brakes acting on the wheels are usedprimarily for steering.

Of course, the same braking systems must be capable of being used whenthe vehicle is being driven on roads. Under such driving conditions, thetwo brake pedals are physically connected to one another, so that theycannot be depressed separately, and symmetrical braking is achieved byhydraulically interconnecting the two braking systems so that the samebraking pressure is applied to the slave cylinders on both sides of thevehicle.

As a result, for driving on normal roads, more braking capacity isavailable to the driver than is needed and in some countries there is alegal requirement for simultaneous braking using the hydrostatic drivesystem which increases the maximum braking force still further.

The availability of an excessively high braking force presents aparticular problem in the case of harvesters that are being driven on aroad in that they risk toppling forwards. This problem is aggravated bythe fact that, when driven on a road, the header of a harvester, that isto say the attachment on the front of the vehicle which is operable tocut and collect the crop, has to be raised and therefore changes theposition of the centre of gravity of the whole vehicle.

OBJECT OF THE INVENTION

The present invention therefore seeks to avoid the risk of anagricultural vehicle toppling forwards when driven on a road withoutreducing its braking capacity during field operation.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the present invention, there isprovided an agricultural vehicle having ancillary equipment that may beraised and lowered relative to the ground with consequential changing ofthe position of the centre of gravity of the vehicle and having abraking system of variable capacity to enable the maximum braking forceto be reduced when the position of the centre of gravity of the vehicleis changed in order to prevent the vehicle from toppling forwards duringmaximum braking.

The braking capacity can be reduced in a variety of ways. A firstpossibility is to provide two slave cylinders on each wheel, both beingactivated under normal conditions and one being disabled when thebraking capacity is to be reduced.

In this second aspect of the invention, there is provided anagricultural vehicle having ancillary equipment that may be raised andlowered relative to the ground with consequential changing of theposition of the centre of gravity of the vehicle and having a hydraulicbraking system comprising a master cylinder and a slave cylinder,wherein the braking system further includes a second slave cylinderassociated with the same wheel as the first slave cylinder and a valvefor isolating the second slave cylinder from the master cylinder whenthe position of the centre of gravity of the vehicle is changed, therebyensuring that the braking force does not exceed a safe limit below whichthe vehicle does not risk toppling forwards when braking.

Another possibility for achieving a variable capacity braking system isto provide a switchable hydraulic amplification stage which operatesduring field use but not during road use.

In this third aspect of the invention, there is provided an agriculturalvehicle having ancillary equipment that may be raised and loweredrelative to the ground with consequential changing of the position ofthe centre of gravity of the vehicle and having a hydraulic brakingsystem comprising a master cylinder and a slave cylinder, wherein thebraking system additionally comprises a pressure amplification stage anda valve having a first position in which the master cylinder isconnected to the slave cylinder by way of the pressure amplificationstage and a second position in which the master cylinder is directlyconnected to the slave cylinder, the pressure amplification stage beingbypassed in the second position of the valve to ensure that the brakingforce does not exceed a safe limit below which the vehicle does not risktoppling forwards when braking.

Other possibilities for a dual capacity or variable capacity brakingsystem will be readily apparent to the person skilled in the art, butthe preferred approach is to selectively limit the braking force bylimiting the pressure in the hydraulic braking circuits during road use.The advantage of this approach is that it involves minimal alteration toexisting braking systems and can therefore if necessary be retrofittedeasily to existing vehicles.

In the fourth and most preferred aspect of the invention, there isprovided an agricultural vehicle having ancillary equipment that may beraised and lowered relative to the ground with consequential changing ofthe position of the centre of gravity of the vehicle and having ahydraulic braking system comprising a master cylinder and a slavecylinder, wherein the braking system further includes a pressure reliefvalve for limiting the hydraulic pressure applied to the slave cylinderwhen the centre of gravity of the vehicle is raised, thereby ensuringthat the braking force does not exceed a safe limit below which thevehicle does not risk toppling forwards when braking.

Preferably, the agricultural vehicle has two hydraulic braking systemswith separate brake pedals each acting on a respective side of thevehicle, so as to enable the vehicle to be steered by the application ofa braking force to only one side of the vehicle and the master cylindersof the two braking systems additionally comprise pressure equalisationports that are connected to one another, each port including anon-return valve that is opened as soon as the associated brake pedal isdepressed, so that equal pressures are applied to the slave cylinders onthe opposites sides of the vehicle when the two brake pedals aredepressed simultaneously.

The pressure relief valve is conveniently connected in this case to thehydraulic line interconnecting pressure equalisation ports of the mastercylinders.

In order to activate and disable the pressure relief valve selectively,a two position valve may suitably be arranged in series with thepressure relief valve, the two position valve serving to isolate thepressure relief valve from the pressure equalisations ports in its firstposition but not in its second position.

The two position valve could simply be a manually operated ON/OFF valvewith an interlock that obliges it to be opened before the two brakepedals can be connected to one another for driving on a road.

It is preferred, however, to provide a normally closed solenoid valvethat is operated automatically in dependence upon the selected driveratio, the speed of the vehicle, the height of the header or any otherparameter indicative of road use, as opposed to field use, of thevehicle.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described further, by way of example, withreference to the accompanying drawing, in which:

FIG. 1 is a block diagram of the hydraulic circuit of a braking systemof an agricultural vehicle constructed in accordance with the first andfourth aspect of the invention,

FIG. 2 is section through a valve of a braking system constructed inaccordance with the second aspect of the invention,

FIG. 3 is a block diagram of part of the hydraulic circuit of a brakingsystem constructed in accordance with the third aspect of the invention,and

FIG. 4 is a block diagram similar to that of FIG. 3 showing the brakingsystem during field use of the vehicle when the brake pedals aredepressed separately.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a braking system comprising two (left and right) mastercylinders 10, 10′ connected to two slave cylinders 12, 12′ by way ofrespective hydraulic pressure lines 14, 14′ to form separate left andright braking circuits. The two braking circuits are identical and onlyone of them needs to be described.

In each of the braking circuits, the master cylinder 10 comprises ahousing 10 a containing a piston 10 b that is moved by means of anactuating rod 10 c connected to a respective brake pedal. When thepiston 10 b is moved by depressing a brake pedal, fluid under pressureis supplied through a port 10 d to the slave cylinder 12, the piston ofwhich acts on a brake pad on a respective side of the vehicle.

Each of the master cylinders 10, 10′ is further connected through a port10 e that incorporates a non-return valve 10 f, to a common reservoir 16at ambient pressure through a line 18. The reservoir maintains thecircuits filled with hydraulic fluid as their volume increases throughwear of the brake pads.

As so far described, each hydraulic circuit is entirely conventional andoperates in the same manner as the braking system of most road vehicles.

Because two separate braking circuits are provided for the left andright side of the vehicle, during field operation the left and rightsides of the vehicle can be braked separately by operating only one orother of the two brake pedals and this allows the vehicle to be steeredby means of its brakes.

Such braking is inappropriate, for obvious reasons, when the vehicle isto be driven on a road. Thus, during road use, the brake pedals aremechanically connected to one another so that they cannot be operatedseparately. One cannot rely on the mechanical coupling of the brakepedals to ensure equal braking on both sides of the vehicle and this isinstead accomplished, in a known manner, by interconnecting thehydraulic circuits. To this end, each of the master cylinders 10 and 10′additionally comprises a pressure equalisation port 10 g thatincorporates a non-return valve 10 h. The piston 10 b has a shoulderwhich acts on the closure member of the non-return valve 10 h as soon asthe brake pedal is depressed to connect the working chamber of thecylinder to a pressure equalisation line 20. If only one of the brakepedals is operated during field use, only one of the non-return valves10 h will be open and high pressure will not be able to flow from theactuated braking circuit to the other. However if both pedals aredepressed, even by unequal amounts, the two circuits will be able tocommunicate with one another through the pressure equalisation line 20to ensure symmetrical braking when the vehicle is in road use.

As so far described, the braking system is known in the context of brakesteered agricultural vehicles. Master cylinders having an additionalpressure equalisation which also incorporate a non-return valve in thepressure equalisation port that is opened as soon as the piston is movedare currently available and their internal construction need nottherefore be described in greater detail herein.

The illustrated embodiment of the present invention comprises a solenoidoperated two position valve 24 and a pressure relief valve 26 arrangedin series with one another in a line 22 that leads from the pressureequalisation line 20 to the line 18 connected to the fluid reservoir 16.

The solenoid valve 24 is shown in its normal position for field usewherein it maintains the pressure equalisation line 20 isolated and doesnot therefore interfere with pressure supplied to the slave cylinders12, 12′.

However, when a parameter is sensed that indicates that the vehiclerisks toppling because it is being driven on a road with the headerraised, the solenoid valve 24 is moved into its other position in whichit connects the pressure equalisation line 20 to the pressure reliefvalve 26.

The pressure relief valve 26 is a spool valve that is acted upon in onedirection by a spring and in the opposite direction by a pilot pressurederived from its intake port. As soon as the pressure applied to therelief valve 26 exceeds a threshold, which as represented by an arrow inthe drawing may be adjustable to suit the vehicle, the valve opens andconnects the pressure equalisation line 20 to the ambient pressure inthe reservoir 16. In this way, the pressure delivered to the slavecylinders 12, 12′ is limited to the value set by the pressure reliefvalve and the risk of the vehicle toppling is avoided.

The signal for operating the solenoid valve 24 may be derived from anysuitable source, for example from a sensor that responds to the selecteddrive ratio, the speed of the vehicle or the height of the header.

It will be seen from the drawing that the additional components requiredto eliminate the risk of the vehicle toppling during road use are thetwo contained within the box drawn in dotted lines, namely the twoposition solenoid valve 24 and the pressure relief valve 26. These twocomponents can be formed as a sub-assembly that may be connected usingonly two hydraulic connections to an existing braking system, thusmaking it possible to modify existing vehicles with relative ease.

Though the embodiment of FIG. 1 is preferred because of the ease ofretrofitting, the capacity of the braking system can be altered in otherways as will now be described with reference to two further embodimentsof the invention, shown in FIG. 2 and in FIGS. 3 and 4, respectively. Inthe case of both these further embodiments, all the components shown inFIG. 1, other than the two valves 24 and 26, are present. These are thecomponents that are conventionally to be found in an agriculturalvehicle that can be steered by asymmetrical braking.

In the case of the alternative embodiment, a valve body 50 as shown inFIG. 2 is provided, in addition to which a second brake calliper (notshown), which incorporates an additional slave cylinder, is provided oneach wheel. Lines 60 and 62 lead to the separate slave cylinders on theleft side of the vehicle and lines 60′ and 62′ lead to separate slavecylinders on the right side of the vehicle. Once again, primed referencenumerals will be used to avoid unnecessary repetition of description.

The valve body 50 shown in FIG. 2 has two intake ports connected to thelines 14, 14′ from the master cylinders 10, 10′ and four output portsconnected to the lines 60, 62, 60′ and 62′. A valve spool 52 is mountedin a bore in the valve body 50 and at each end the spool is acted uponby a spring 55, 55′ and the pressure in a control chamber 54, 54′ thatcommunicates with one of the intake ports. When equal pressures areapplied to the two control chambers 54, 54′, the spool 52 adopts itsillustrated position. Here the lines 14 and 14′ are connected to thelines 60, and 60′ through passages 66 and 66′ in the body 50 while thetwo further passages 68 and 68′ that lead to the lines 62 and 62′ areclosed by the valve spool 52. Thus, when both brake pedals are depressedduring road use of the vehicle, only one set of brake pads acts on eachside of the vehicle.

When only the left side is braked, the pressure in the control chamber54 will move the valve spool 52 to the right as viewed, allowing fluidto flow through the passage 68 to the line 62 leading to the slavecylinder of the second calliper and two sets of brakes will be activatedon the left side of the vehicle but none on the right, causing thevehicle to be steered to the left. To allow hydraulic fluid to bedrained from the second slave cylinder when the brake pedal is releasedand the passage 68 is again closed by the valve spool 52, a non-returnvalve 64 is provided which allows fluid to flow from the line 62 to theline 14 only in the direction that reduces the braking force on thewheels.

By operation of the valve body 50, it is ensured that maximum brakingforce can still be applied when braking only one wheel, by means of thefirst and second slave cylinders. The braking force will however bedivided by two when braking both wheels simultaneously because only thefirst slave cylinder on the left and right wheel is used due to an equalpressure being applied to both sides of the valve spool 52.

The embodiment of FIGS. 3 and 4 comprises a valve 30 connected to thelines 14, 14′ of FIG. 1 and two pressure amplifiers 38 and 38′ eachconnected to a respective one of the slave cylinders 12 and 12′ in FIG.1.

The valve 30 is generally similar to the valve 50 in FIG. 2 and hascontrol chambers 34 and 34′ connected to the lines 14 and 14′ from themaster cylinders. As in the case of the valve spool 52 in FIG. 2, thespool 32 of the valve 30 adopts a central position, shown in FIG. 3,when both brake pedals are depressed during road operation and an endposition, shown in FIG. 4, when only one brake pedal is depressed duringfield operation.

During road operation, the master cylinder lines 14 and 14′ areconnected to the slave cylinders 12 and 12′ through lines 42 and 42′that bypass the pressure amplification stages 38 and 38′ so that thelatter have no effect. On the other hand, when the spool 32 moves to itsend position shown in FIG. 4 because only the left brake pedal isdepressed, the passage 42 is blocked by a land of the spool 32 andinstead the pressure from the master cylinder line 14 is applied throughthe passage 40 to the amplification stage 38 which increases thepressure applied to the slave cylinder 12 by a factor equal to the ratioof the surface areas of the opposite ends of its piston. Similarly, ondepressing the right brake pedal, the master cylinder line 14′ isblocked by another land of the spool 32, preventing pressure to betransmitted to the right slave cylinder 12′.

Accordingly, when driving on the road and depressing both brake pedalssimultaneously, the spool 32 is forced to stay in the middle of thevalve 30, because of the equal pressure in the master cylinder lines 14and 14′.

However, because of the pressure amplification stages 38 and 38′, thesystem is able to create a higher braking force when the left or rightbrake is used alone. The system is dimensioned such that the brakingforce is within safe braking limits when driving on the road.

1. An agricultural vehicle having ancillary equipment that may be raisedand lowered relative to the ground with consequential changing of theposition of the centre of gravity of the vehicle and having a hydraulicbraking system comprising a master cylinder and a slave cylinder,wherein in that the braking system further includes a pressure reliefvalve for limiting the hydraulic pressure applied to the slave cylinderwhen the position of the centre of gravity of the vehicle is changed,thereby ensuring that the braking force does not exceed a safe limitbelow which the vehicle does not risk toppling forwards when braking,having two hydraulic braking systems with separate brake pedals eachacting on a respective side of the vehicle so as to enable the vehicleto be steered by the application of a braking force to only one side ofthe vehicle and wherein the master cylinders of the two braking systemsadditionally comprise pressure equalization ports that are connected toone another, each port including a non-return valve that is opened assoon as the associated brake pedal is depressed, so that equal pressuresare applied to the slave cylinders on the opposite sides of the vehiclewhen the two brake pedals are depressed simultaneously.
 2. Anagricultural vehicle as claimed in claim 1, wherein the pressure reliefvalve is connected to at least one hydraulic line interconnecting thepressure equalization ports of the master cylinders.
 3. An agriculturalvehicle as claimed in claim 2, wherein a two position valve is arrangedin series with the pressure relief valve, the two position valve servingto isolate the pressure relief valve from the pressure equalizationports in its first position but not in its second position.
 4. Anagricultural vehicle as claimed in claim 3, wherein the two positionvalve is biased by a spring into its first position and is moved intoits second position by a solenoid when the maximum braking force is tobe limited.
 5. An agricultural vehicle as claimed in claim 4, whereinthe two position valve is moved into its second position in dependenceupon the selected road speed drive ratio.
 6. An agricultural vehicle asclaimed in claim 5, wherein a threshold pressure at which the pressurerelief valve opens is adjustable.
 7. An agricultural vehicle as claimedin claim 4, wherein the two position valve is moved into its secondposition in dependence upon the speed of the vehicle.
 8. An agriculturalvehicle as claimed in claim 7, wherein a threshold pressure at which thepressure relief valve opens is adjustable.
 9. An agricultural vehicle asclaimed in claim 4, wherein the two position valve is moved into itssecond position by means responsive to the height of the ancillaryequipment.
 10. An agricultural vehicle as claimed in claim 9, wherein athreshold pressure at which the pressure relief valve opens isadjustable.
 11. An agricultural vehicle as claimed in claim 4, wherein athreshold pressure at which the pressure relief valve opens isadjustable.
 12. An agricultural vehicle as claimed in claim 3, wherein athreshold pressure at which the pressure relief valve opens isadjustable.
 13. An agricultural vehicle as claimed in claim 2, wherein athreshold pressure at which the pressure relief valve opens isadjustable.
 14. An agricultural vehicle as claimed in claim 1, wherein athreshold pressure at which the pressure relief valve opens isadjustable.
 15. An agricultural vehicle comprising: a braking systemconfigured to at least reduce a speed of travel of the agriculturalvehicle over a ground; at least one piece of equipment that isadjustable between a raised position and a lowered position relative tothe ground and, wherein adjustment between the raised position and thelowered position causes a corresponding adjustment in a position of acenter of gravity of the agricultural vehicle; and a braking controlsystem: a master cylinder and a slave cylinder configured to control aflow of hydraulic fluid through the braking system to reduce the speedof travel of the agricultural vehicle; a pressure relief valveconfigured to control a pressure of the hydraulic fluid applied to theslave cylinder changes the position of the center of gravity of theagricultural vehicle to reduce a probability of the agricultural vehicletoppling forward during braking; further comprising another brakingcontrol system and at least two brake pedals and wherein each of the atleast two brake pedals is configured to control braking on a respectiveside of the agricultural vehicle to enable the vehicle to be steered bythe application of a braking force to only one side of the vehicle.